1. Field of the Invention
The present invention relates to a method and apparatus for detecting a noise disappearance, wherein output signals from a detector are received successively and a decreased noise component concerned with process vibrations and included in the output signal due to a fault of a plant (sticking of a valve, blocking of a piping, etc.) or a trouble in an instrument itself is detected, thereby detecting the plant fault or the trouble in the instrument itself.
2. Description of the Related Art
It has heretofore been difficult to detect a fault of the plant or the instruments at an early stage because, in most cases, operators did not sense the fault of the instruments, etc. until serious faults occur in a process. Particularly, after adoption of the DCS instrumentation, it has become difficult to understand a deviation with time of detected values using only digital values, and further, to detect a fault. In addition, it is difficult to provide a device exclusively used for sensing the fault from the viewpoint of economics.
Therefore, there has been proposed a device for detecting almost no change in an output signal from a detector provided in a plant, and then giving a predetermined alarm indicating the occurrence of a fault in the plant or the detector.
As shown in FIG. 4 for example, in this device, if the output signal falls into a preset abnormality determining value range having upper and lower limits (between HL and LL), the fact that the output value continues to be hardly changed is determined to be an abnormality by a determining system in which a difference of the output value from a certain reference value becomes continuously small.
However, when the modes of abnormality occurrence in the plant due to faults of the detectors, blocking of the piping and the like are taken into consideration, it is not sufficient to merely look for a state of no change in the output signal from the detector as in the conventional techniques. More specifically, with actual abnormalities, there are many cases where, even if the output signal of the detector fluctuates by increasing or decreasing, noise components which should be intrisically included in the fluctuation of the signal decrease due to the abnormality, and it is unavoidable that the abnormality described above cannot be detected by the conventional technique. This is because the reference value during the occurrence of the abnormality is used in a fixed manner, that is, a detected value is merely compared with the reference value which is not renewed or updated.
Fluctuations of the process are divided into two types. One type of fluctuation of the process involves low frequency waves which occur in a case where the operating conditions of the process are changed or disturbances take place in the process. In this case, the value of the process changes relatively moderately. On the other hand, even when the stable operation is performed with no change being given to the process, if a liquid surface is shaken or pulsations in the flow rate normally occur due to conveyance by a pump, then there occurs the other type of fluctuation of the process, which involves high frequency waves such as noises.
When the latter fluctuations are observed, it may be said that the output value is normal. Accordingly, in order to perform noise disappearance detecting with high accuracy, it is desirable that the former low frequency fluctuations and the latter high frequency fluctuations be monitored separately from each other.
As a specific example, the assumption is made that a differential pressure type level meter used as a detector is provided in a plant and detects a level of a liquid surface in a vessel. The above-described level meter and vessel are connected to each other through a conduit.
In general, it might seem that the output signal from the level meter falls into a state of no change if the interior of the conduit becomes blocked. In practice, however the pressure in the conduit is varied with time due to the condensation and contraction of the vapor phase in the conduit and, in many cases, the output signal from the detector continues to increase or decrease. And yet, even in these cases, the high frequency vibration components included in the output signal decrease, but there has been presented the problem that the noise disappearance cannot be detected by the conventional technique where the output signal shows the incrase or decrease.